Don't return a mutable interval from IntervalSet.intersection()

Instead, always take the subset, which creates a new interval.
Also adds a small optimization by moving the 'if orig' check outside the
loop.

nilmdb/server/interval.pyx

@@ -286,23 +286,18 @@ cdef class IntervalSet:
         (potentially) subsetted to make the one that is being
         returned.
         """
-        if not isinstance(interval, Interval):
-            raise TypeError("bad type")
-        for n in self.tree.intersect(interval.start, interval.end):
-            i = n.obj
-            if i:
-                if i.start >= interval.start and i.end <= interval.end:
-                    if orig:
-                        yield (i, i)
-                    else:
-                        yield i
-                else:
-                    subset = i.subset(max(i.start, interval.start),
-                                      min(i.end, interval.end))
-                    if orig:
-                        yield (subset, i)
-                    else:
-                        yield subset
+        if orig:
+            for n in self.tree.intersect(interval.start, interval.end):
+                i = n.obj
+                subset = i.subset(max(i.start, interval.start),
+                                  min(i.end, interval.end))
+                yield (subset, i)
+        else:
+            for n in self.tree.intersect(interval.start, interval.end):
+                i = n.obj
+                subset = i.subset(max(i.start, interval.start),
+                                  min(i.end, interval.end))
+                yield subset
 
     cpdef intersects(self, Interval other):
         """Return True if this IntervalSet intersects another interval"""